This invention relates to the field of electrical capacitors, and in particular, it concerns a high Q monolithic metal-insulator-metal (MIM) capacitor.
The elements which limit the performance of small, monolithic filters are the capacitors rather than the inductors. MIM capacitors would be ideal elements for monolithic filters if it were not for the low Q of the thin film dielectrics. Thin film capacitors are limited to Q's of typically 20 to 50 due to the low loss tangents of thin film dielectric materials, such as sputtered SiO.sub.2. The Q of a MIM capacitor would be approximately 800 at X-band if it were controlled by metal losses alone. This can have a significant impact on the performance of a filter design. For example, a typical elliptic filter with series and shunt capacitor Q's of 25 may have an insertion loss of 4 dB over a passband of 9-10 GHz, while a similar filter with capacitor Q's of 800 may have an insertion loss of only 0.6 dB.
A further need for high Q capacitors is in the area of superconducting circuits. The increased use of thin film superconductors highlights the need for small, lumped element, low loss, monolithic filters. Such superconducting filters will not be possible without eliminating the lossy capacitor dielectric as a fundamental limitation.